1. Field of the Invention
This invention is concerned with corrosion inhibition, and in particular with detection of corrosion inhibiting species released from a coating.
2. Description of the Related Art
Corrosion of a metal or alloy generally involves oxidation of metal atoms (M), which may dissolve in a liquid, or form a solid compound such as a metallic oxide. The driving force for such metallic corrosion is typically oxygen reduction, which consumes the electrons generated during oxidation of the metal atoms. The metal oxidation reaction and the oxygen reduction reaction form an electrochemical couple defined by the general half reactions:M−>Mx++xe−  (1)andO2+2H2O+4e−−>4OH− (in alkaline solutions)  (2)orO2+4H++4e−−>2H2O (in acidic solutions)  (3)For this couple, the anodic reaction is metal oxidation and the cathodic reaction is oxygen reduction. Although not shown for these overall half reactions, oxygen reduction generally proceeds via a peroxide intermediate, which may be the reaction end product under some conditions.
The oxygen reduction reaction, which typically provides the driving force for metallic corrosion, is sustained only in the presence of a catalyst. Oxygen reduction tends to be catalyzed by more noble metals, e.g., osmium, ruthenium, iridium, rhodium, platinum, palladium, gold, silver and copper. Less noble metals tend to react rapidly with oxygen to form a relatively thick surface oxide layer that inhibits further oxygen reduction. Oxygen reduction is also catalyzed by some forms of carbon and some carbon compounds.
An impurity or a constituent added to impart desirable physical or chemical properties to an alloy may serve as an oxygen reduction catalyst, reducing the corrosion resistance of the alloy. For example, high-strength aluminum alloys often contain copper, which tends to segregate at aluminum grain boundaries and form catalytic sites at the alloy surface. In the presence of liquid water or an aqueous solution, the copper sites may serve as cathodes at which oxygen is reduced so that electrons are withdrawn from the alloy, causing aluminum metal to be electrochemically oxidized to aluminum ions.
Corrosion inhibitors, such as hexavalent chromium ions, are often added to paints, primers and other coatings to suppress corrosion of the underlying metal. It is known in the art that such corrosion inhibitors typically function by adsorbing or reacting at catalytic cathode sites so that the oxygen reduction reaction is suppressed [G. O. Ilevbare and J. R. Scully, J. Electrochem. Soc. 148 (5), B196 (2001); and M. Kendig and S. Jeanjaquet, J. Electrochem. Soc. 149 (2), B47 (2002)]. Corrosion tends to be more effectively suppressed if the inhibitor is released from the coating slowly, or as needed to inhibit corrosion at a damaged area in the coating. When the inhibitor becomes depleted in the coating, the extent of corrosion protection provided by the coating is diminished. Furthermore, the effectiveness of a corrosion inhibitor typically depends on the type of coating in which the inhibitor is included and may be degraded by variations in the coating constituents, formulation and application process.
A rapid and effective method for evaluating the corrosion inhibiting activity of a coating is needed to efficiently identify effective combinations of corrosion inhibitors, coatings and application processes. This need is particularly important since the widely used hexavalent chromium inhibitor has been found to be environmentally unacceptable. Such a coating evaluation method could also be used for quality control to ensure that the coating application process consistently provides coatings with high corrosion inhibiting activity. There is also an important need for a rapid, portable, inexpensive and non-destructive means for periodically evaluating the corrosion inhibiting activity of coatings during use to determine when coating renewal or other preventative maintenance is needed. Reliable information concerning the corrosion inhibiting activity of a coating could be used to save time, effort and expense by deferring coating renewal until needed while avoiding corrosion damage to the substrate that might require more costly repairs or result in substrate failure. However, prior art methods for evaluating corrosion inhibiting coatings are inadequate.
The effectiveness of a coating in suppressing corrosion of a substrate is typically determined by standard salt spray or salt fog tests (ASTM B117), in which a coated substrate is exposed to a corrosive environment and subsequently examined for signs of substrate corrosion. The corrosive environment typically includes air and a saline solution (salt fog may also include other corrosive species, such as SO2 gas). A coating defect may be intentionally introduced (by scribing, for example) to assess the effectiveness of damaged or defective coatings for inhibiting corrosion of the substrate. Typical evaluation criteria are the number of coating pores or defects discolored by corrosion products, and the degree of discoloration. Such evaluations are subjective since the pore/defect size and the extent and the nature of the discoloration may vary widely. These tests also involve bulky aging chambers and long exposure times (typically 168 to 336 hours for anodized coatings and conversion coatings, and 1000 to 2000 hours for paint primers), and are costly to perform. Furthermore, the harsh test conditions employed may not yield results that predict the performance of the coating in actual use. In addition, salt spray and salt fog tests are destructive so that they cannot be used for periodic evaluation of a coating in service to determine the need for renewal of the coating or other preventative maintenance.
Electrochemical methods, such as electrochemical impedance spectroscopy (EIS), are often used to measure corrosion rates of metals and alloys, with and without corrosion protective coatings. These methods involve direct measurement of the current associated with the substrate corrosion process and are thus insensitive to the corrosion inhibiting activity of the coating until significant substrate corrosion has already begun. Consequently, electrochemical methods of the prior art cannot be used to evaluate the corrosion inhibiting activity of a coating prior to depletion of the inhibitor. In addition, corrosion rate measurements are substantially insensitive to the small amounts of corrosion inhibitor typically released from a corrosion protective coating.